This invention relates to a radial tire and, more particularly, to a radial tire in which each sidewall covering and protecting the side face of the carcass is composed of a plurality of layers differing in rubber composition.
Prior art pneumatic tires generally have sidewalls made of a rubber composition comprising a rubber having a high degree of unsaturation, the main chain of which has a high double bond content, such as natural rubber, polyisoprene, polybutadiene, or styrene-butadiene copolymer. Since the double bonds in the main chain of the molecule of such rubber are apt to react with ozone, leading to depolymerization, when the tire is exposed to intense ultraviolet rays or to a high concentration of ozone in air, cracking, namely the so-called ozone cracking, occurs.
In particular, radial tires with their carcass reinforced with belts made of a cord of high Young's modulus, such as a steel cord, at the top portion of the carcass are susceptible to ozone cracking since they have sites of discontinuity in rigidity from the tread portion to each side portion. Thus, when in contact with the ground and under loading, the tread portion is constrained by the high-rigidity belts and the resulting shear strain is small but the shear stress increases accordingly. On the other hand, in the side portions which are not constrained by rigidity, the shear stress decreases but, instead, the shear strain increases markedly. In such strained state, the development of ozone cracks on the rubber composition surface is accelerated in an exponential manner.
For preventing such cracking, an antiozonant, for example an amine type antioxidant or a wax, is generally incorporated in the rubber composition constituting the sidewalls.
However, radial tires the sidewalls of which are made of a rubber composition containing such an antiozonant are subject to blooming of the antiozonant on the sidewall surface during storage. Such blooming makes them look as if they are covered with dust. Furthermore, amine type antioxidants readily turn brown upon exposure to air and/or sunlight, so that the thin antiozonant layer formed on the sidewall surface as a result of blooming becomes brown. This masks the original black color of the tires and spoils their appearance, reducing their commercial value.
In addition, amine type antioxidants are carried away by rain water, among others, so that their content in sidewalls decreases. It is difficult to maintain their cracking-preventing effect stably.
Therefore, investigations were made to construct the sidewalls with a rubber composition containing an antiozonant, which may possibly cause blooming and staining of the sidewalls, in a reduced amount by using a rubber having a low unsaturated bond content, or a low degree of unsaturation, and thus having stronger ozone resistance by itself (e.g. U.S. Pat. No. 4,224,196; Japanese Kokai Patent Publication No. 59-14502; Japanese Kokai Patent Publication No. 62-143955).
However, rubbers having a low degree of unsaturation are curable at a slower rate as compared with diene rubbers (rubbers having a high degree of unsaturation) generally used in carcass and tread formation. Therefore, even when the rate of curing of the former is adjusted so as to become comparable to the rate of curing of the latter by using a vulcanization accelerator, migration of the accelerator occurs at the interface between the former and the latter, so that the rate of curing becomes faster locally and the period over which both the rubber species can diffuse through each other becomes shortened. As a result, the adhesion between them becomes poor. Such problem cannot be solved even by the means proposed in Japanese Kokai Patent Publication No. 59-14502, which comprises constructing the sidewalls each as a double-layer structure having an inner layer and an outer layer using a rubber composition containing a large amount of a rubber having a low degree of unsaturation for the construction of the outer layer and a diene rubber for the construction of the inner layer. Radial tires in which a rubber having a low degree of unsaturation is used for the formation of the sidewall thus have never been put to practical use.